1. Field of the Invention
The present invention relates to a novel method of manufacturing electron-emitting devices, and methods of manufacturing electron sources and image-forming apparatus based on the novel manufacturing method of electron-emitting devices.
2. Related Background Art
There are hitherto known two major types of electron-emitting devices; i.e., thermionic cathode type electron-emitting devices and cold cathode type electron-emitting devices. Cold cathode type electron-emitting devices include the field emission type (hereinafter abbreviated to FE), the metal/insulating layer/metal type (hereinafter abbreviated to MIM), the surface conduction type, etc. Examples of FE electron-emitting devices are described in, e.g., W. P. Dyke & W. W. Dolan, "Field emission", Advance in Electron Physics, 8, 89 (1956) and C. A. Spindt, "PHYSICAL properties of thin-film field emission cathodes with molybdenium cones", J. Appl. Phys., 47, 5248 (1976).
One example of MIM electron-emitting devices is described in, e.g., C. A. Mead, "Operation of Tunnel-Emission Devices", J. Appl. Phys., 32, 646 (1961).
One example of surface conduction electron-emitting devices is described in, e.g., M. I. Elinson, Radio Eng. Electron Phys., 10, 1290, (1965).
Surface conduction electron-emitting devices operate based on a phenomenon that when a thin film of small area is formed on a substrate and a current is supplied to flow parallel to the film surface, electrons are emitted therefrom. As to such surface conduction electron-emitting devices, there have been reported, for example, one using a thin film of SnO.sub.2 by Elinson cited above, one using an Au thin film G. Dittmer: Thin Solid Films, 9, 317 (1972)!, one using a thin film of In.sub.2 O.sub.3 /SnO.sub.2 M. Hartwell and C. G. Fonstad: "IEEE Trans. ED Conf.", 519 (1975)!, and one using a carbon thin film Hisashi Araki, et. al.: Vacuum, Vol. 26, No. 1, 22 (1983)!.
As a typical example of those surface conduction electron-emitting devices, FIG. 27 schematically shows the device configuration proposed by M. Hartwell, et. al. in the above-cited paper. In FIG. 27, denoted by reference numeral 1 is a substrate. 4 is an electro-conductive thin film formed of, e.g., a metal oxide thin film made by sputtering into an H-shaped pattern, in which an electron-emitting region 5 is formed by energization treatment called energization forming (described later). Incidentally, the spacing L between opposed device electrodes is set to 0.5-1.0 mm and the width W' of the electro-conductive thin film is set to 0.1 mm.
The configuration of surface conduction electron-emitting devices is not limited to the H-pattern mentioned above. By way of example, a surface conduction electron-emitting device may be constructed such that opposite portions of the H-pattern are formed as electrodes and an electro-conductive thin film is formed to interconnect the electrodes. In this configuration, the electrodes and the electro-conductive thin film may be different in material and thickness from each other.
In those surface conduction electron-emitting devices, it has been customary that, before starting the emission of electrons, the electro-conductive thin film 4 is subjected to energization treatment called energization forming to form the electron-emitting region 5. Specifically, the term "energization forming" means applying a DC voltage or a voltage gradually increasing at a very slow rate of about 1 V/min, for example, across the electro-conductive thin film 4 to locally destroy, deform or denature it, to thereby form the electron-emitting region 5 which has been transformed into an electrically high-resistance state. In the electron-emitting region 5, a fissure or fissures are produced in part of the electro-conductive thin film 4 and electrons are emitted from the vicinity of the fissure(s) when a voltage is applied to the electro-conductive thin film 4 so that a current flows through the device.
The surface conduction electron-emitting device is simple in structure and easy to manufacture, and hence has an advantage that a number of devices can be formed into an array having a large area. Therefore, a variety of application studies with a view of utilizing such advantageous features of the surface conduction electron-emitting device have, been conducted. Typical application fields includes, e.g., charged beam sources and display devices. As one example of applications in which a number of surface conduction electron-emitting devices are formed into an array, there is proposed an electron source that, as described later in detail, surface conduction electron-emitting devices are arrayed in parallel, opposite ends of the individual devices are interconnected by two wires (called also common wires) to form one row, and a number of rows are arranged to form a matrix pattern. (See, e.g., Japanese Patent Application Laid-Open No. 64-031332, No. 1-283749 and No. 2-257552). In the field of image-forming apparatuses such as display devices, particularly, plane type display devices using liquid crystals have recently become popular instead of CRTs, but they are not self-luminous and have a problem of requiring backlights or the like. Development of self-luminous display devices have therefore been desired. An image-forming apparatus is proposed in which an electron source having an array of numerous surface conduction electron-emitting devices and a fluorescent film radiating visible light upon impingement of electrons emitted from the electron source are combined with each other to form a display device. (See, e.g., U.S. Pat. No. 5,066,883).
In the known manufacture method, the forming step of forming the electron-emitting region is performed by applying a voltage to the electro-conductive thin film as explained above. With the Joule heat generated by the voltage applied, the electro-conductive thin film is partly denatured and deformed into a highly resistant state. That method has, however, had problems as noted below.
(1) Problem on control of position and shape of electron-emitting region:
The position where the electro-conductive thin film is denatured and deformed depends on various factors, but an important factor is in which part of the electro-conductive thin film the temperature is most substantially raised due to the heat generated.
If the electro-conductive thin film is uniform and the device electrodes have good symmetry, it is believed that the temperature is most substantially raised just at the middle between the electrodes. In practice, however, various factors bring about non-uniformity in the electro-conductive thin film, and symmetry of the electrode shape is often not satisfactory when the electrodes are formed by printing or the like. Also, it is believed that a high-resistance portion serving as the electron-emitting region is formed through a complex process in which when one high-resistance portion is formed in part of the electro-conductive thin film, current distribution is changed correspondingly, whereupon a next high-resistance portion is formed in the part in which the current is newly concentrated. Due to a slight disturbance, therefore, the shape of the electron-emitting region may have different widths depending on parts or may extend in a zigzag direction. This poses a difficulty in providing even device characteristics. In particular, when an electron source comprising an array of numerous electron-emitting devices and an image display device using the electron source are fabricated, the amount of emitted electrons and the brightness of pictures may vary.
For example, when an electron source is employed in an image display device having a large area, it is generally desired to form wiring and electrodes by screen printing from the standpoint of production techniques. In this case, however, the spacing between device electrodes opposed to each other is fairly wider than that based on film-forming by vacuum evaporation or sputtering and patterning by photolithography. This may lead to a problem that the electron-emitting region is more liable to extend in a zigzag direction.
(2) Problem on current capacity of wiring due to large forming current:
The step of the energization forming requires a much greater current than during the normal operation as an electron-emitting device. In particular, when an electron source comprising an array of numerous electron-emitting devices is fabricated, the forming treatment is generally carried out on a plurality of devices at a time (e.g., for each row of a matrix pattern of devices). In this case, it is required to flow a considerably greater current than when the electron-emitting devices are normally driven, and hence the wiring is required to have a current capacity for the current supplied. But once the forming treatment is completed, the current capacity actually required in the normal operation is reduced to a much lower level. Therefore, if such a large difference in the current capacity is eliminated, merits from the standpoint of production techniques are expected in points of, e.g., enabling a narrower width of the wiring and increasing the degree of freedom in apparatus design.
Further, because a great current flows through the wiring, a voltage drop is so increased that the state resulting from the forming treatment may be varied in the direction of the wiring to produce a systematic distribution in characteristics of electron emission.
To solve the problems as mentioned above, there has been a demand for establishing a novel method of manufacturing electron-emitting devices.